# Explain why a large multicellular animal, such as a fish, requires a specialised exchange surface for gas exchange, whereas a single-celled organism such as Amoeba does not.

> Edexcel GCSE Biology (1BI0) — 8.1 Gas exchange · Explain · 4 marks

## Mark scheme (4 marks)

1. A single-celled organism has a large surface area to volume ratio
2. So diffusion alone / across the cell surface membrane is sufficient to supply all parts of the cell with oxygen (and remove carbon dioxide)
3. A large multicellular organism has a small surface area to volume ratio
4. So diffusion across the body surface is too slow / insufficient to supply all cells with oxygen, requiring a specialised exchange surface to increase the rate of gas exchange

## Key terms

- [single-celled organism](https://www.gradenine.co.uk/glossary/single-celled-organism)
- [exchange surface](https://www.gradenine.co.uk/glossary/exchange-surface)

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-large-multicellular-animal-1250c233) · Published by Druglandscape Ltd.